The present invention relates to transmissions and in particular to a gear shifting arrangement for a key shift transmission.
In key shift transmissions, at least one forward and one reverse gears forming a change-speed gear set are assembled side by side on a shift in constant running mesh with each other. The shifting arrangement drivingly couples either the forward or the reverse gear to the shaft one at a time to provide different directional motion. In one known type of shifting arrangement for this type of transmission, a key is axially slid into the groove of the shaft which both the forward and reverse gear are assembled to selectively couple one of the gears to the shaft. Normally, a neutral spacer collar is provided between the forward and the reverse gear.
It is conventional to mount the reverse gear both in close proximity to a plurality of forward gears or remote from a plurality of forward gears. U.S. Pat. No. 3,812,735 illustrates the technique of mounting the reverse gear remote from the forward gear and U.S. Pat. No. 4,480,501 illustrates the technique of mounting the reverse gear in close proximity to the forward gear so that the biased end of the key is used to engage and disengage reverse gear, thereby eliminating double clutching necessitated by using the blunt end of the key to engage reverse gear as disclosed by U.S. Pat. No. 3,812,735. As disclosed by U.S. Pat. No. 4,480,501, because the forward gears and reverse gears are in close proximity with each other, a positive neutral position collar is disposed on the shaft between the forward gears and reverse gears to give the user a positive indication or "feel" of the transmission or transaxle being in a neutral gear position. This positive neutral position is provided by a recess radially outwardly disposed in the inner surface of the collar which is conformed to the shape of the biased end of the key.
In one embodiment of the transaxle disclosed in U.S. Pat. No. 4,480,501, the neutral collar has slot-shaped recess which captures the biased end of the key when it is shifted between the forward gears and the reverse gears, while in a second embodiment, the collar has a circular recess which also captures the biased end of the key when it is positioned between the forward gears and reverse gear.
As shown in FIGS. 1 and 1a of U.S. Pat. No. 4,480,501, a neutral collar or spacer is positioned between the single forward and the single reverse gear of a belt driven small lawn and garden tractor. While the neutral spacer as illustrated was initially satisfactory to provide a means for shifting the key from neutral to either forward or reverse or from forward through neutral to reverse, or vice versa, problems developed relating to the amount of effort required by an operator to push or pull the key out of either forward or reverse into the neutral collar area, particularly where there was some belt drag to the transmission input, and when the operator shifted gears without utilizing the clutch. The prior neutral collar design engaged the keys in such a manner that wear areas which developed on the key after a period of time caused the key to disengage from the gear pockets without the operator moving the control lever.
As the key shift transmission was repeatedly shifted between forward and reverse without engaging the clutch, excessive wear on the key caused the key when engaged in the gear pocket to have a tendency to frequently disengage. Once this tendency developed, the key required repair which could only be accomplished by tearing down the transmission and replacing the keys, a rather time consuming and expensive operation.
Specifically, after a number of shifts without utilization of the clutch, laboratory testing indicated that the design of these prior neutral collars caused the key tip surface engaging the gear pocket do become so severely worn from impact with the gear pocket that the key tended to fly out of engagement with the gear pocket for a short period until the next pocket rotated into engagement with the key thereby causing a jerking action. This tendency for the key tip to disengage appears to result from hammering the sides of the key tip into a slope which translates in the pocket. When sloped, the key tip cannot carry the torque created by the engagement with the pocket because with side loading, the slope acts like a ramp and allows the key tip to kick out of the pocket.
At this point, the transmission must be disassembled and, as a minimum, the key replaced. The gear pockets and the shaft must also be reviewed for wear resulting of this severe impact and replaced if excessively worn.
Accordingly, there is a need for an improved key shift transmission having a neutral collar designed such that the above mentioned problems are overcome. Specifically, an improved neutral collar design should provide for a positive neutral feel as well as requiring minimum effort to move the key between forward and reverse gears particularly when there is some belt drag through the transmission input; and should reduce wear on the key such that the key will not disengage from the gear pocket unless positively moved by the control lever, despite excessive clutchless shifting, thereby significantly reducing the frequency of key, gear and shaft replacement thereby enhancing the serviceability and reliability of the entire transmission.